1. Field of the Invention
The present invention relates to information communication systems. More particularly, the present invention relates to a system and method for estimating bandwidth requirements of and allocating bandwidth to communication devices operating in a network.
2. Background Information
A conventional wireless local area network (WLAN) is based on a cellular architecture in which the system is divided into cells. Each cell, referred to as a Basic Service Set (BSS), is controlled by a Base Station, referred to as an Access Point (AP). The APs of the cells are connected to each other through a suitable type of backbone, referred to as a Distribution System (DS). The entire interconnected WLAN, including the different cells, their respective APs and the DS is referred to as an Extended Service Set (ESS).
One or more clients or stations within a cell can communicate using a carrier sense multiple access with collision avoidance (CSMA/CA) protocol for medium access control. In a CSMA/CA protocol, a station desiring to transmit senses the channel. If the channel is busy (i.e., some other station is transmitting), then the station will defer its transmission to a later time. If the channel is sensed to be free at that time, then the station is allowed to transmit. The CSMA/CA protocol works well when the channel is not heavily loaded, allowing the stations to transmit with minimal delay. However, two or more stations may transmit at the same time, causing collisions, if the stations sense that the channel is free and decide to transmit at once.
To overcome such problems, conventional WLANs use a collision avoidance mechanism with a positive acknowledgment scheme. In such a scheme, a station desiring to transmit will sense the channel. If the channel is busy, then the station will defer transmission. If the channel is free for a specified amount of time, then the station (the “sending station”) is allowed to transmit. The receiving station will check the cyclic redundancy code (CRC) of the received frame and send an acknowledgment frame (ACK). If the sending station receives the ACK, then no collision has occurred. However, if the sending station does not receive the ACK, then the sending station will retransmit the frame until it is acknowledged or thrown away after a given number of retransmissions. However, in a WLAN, all stations may not be able to hear each other. Thus, a station may desire to transmit and senses the channel to be free, although the channel may not necessarily be free.
To reduce the probability of two or more stations colliding because they cannot hear each other, conventional WLANs use a virtual carrier sense mechanism. In such a mechanism, a station that desires to transmit will first transmit a short control frame, referred to as a Request to Send (RTS). The RTS includes the source, the destination and the duration of the frame to be transmitted and the respective ACK. If the channel is free, the destination station will respond with a response control frame referred to as a Clear To Send (CTS). The CTS includes the same duration information as the RTS. All stations receiving either the RTS and/or CTS will set their respective virtual carrier sense indication, referred to as Network Allocation Vector (NAV), for the given duration, and will use this information when sensing the channel. The virtual carrier sense mechanism reduces the probability of a collision in the receiver area by a station that cannot be heard by the transmitter to the short duration of the RTS transmission, because the station will hear the CTS packet and “back off” the channel until the end of the transaction. A backoff method is used to resolve contention between different stations desiring access to the channel. A binary exponential backoff algorithm requires each station, when the channel is busy, to choose a random number between zero and a given number, and wait for this number of time slots before accessing the channel, checking whether another station has accessed the channel before transmitting.
The above discussion applies to the Distributed Coordinated Function (DCF) mode. In the DCF mode, there is no central control, and stations compete for transmission time. In contrast, in the Point Coordinated Function (PCF) mode, the AP polls the stations, inquiring whether any of the stations have frames to transmit. Since transmission order is completely controlled by the AP in PCF mode, no collisions occur. In PCF mode, the AP broadcasts a beacon frame periodically. The beacon frame contains various system parameters. The beacon frame also invites stations to sign up for polling service. Once a station has signed up for polling service at a certain rate, the station is allocated a certain fraction of the total bandwidth available for transmission.
Conventional WLANs use an interframe time interval to allow the DCF and PCF modes to coexist within a cell. After a frame is transmitted, a given amount of dead time is required before any station can transmit a frame. The shortest interval is the Short InterFrame Spacing (SIFS) time. SIFS is used to allow stations in a single dialog the chance to go first, including letting the receiver send a CTS to respond to a RTS, letting the receiver send an ACK for a frame fragment or full data frame, and letting the sender of a fragment burst transmit the next fragment without having to send a RTS again. There is always exactly one station that is entitled to respond after a SIFS interval. If the station fails to make use of its chance to transmit and a PCF InterFrame Spacing (PIFS) time elapses (equal to SIFS plus a specified slot time), the AP may send a beacon frame or a poll frame. Such a mechanism allows a station sending a data frame or frame fragment sequence to finish its frame without other stations interfering, but gives the AP a chance to access the channel when the previous sender is done without having to compete with the stations. If the AP has nothing to transmit and a DCF InterFrame Spacing (DIFS) time elapses (equal to PIFS plus one timeslot), any station may attempt to acquire the channel to send a new frame. Contention rules apply, and the binary exponential backoff algorithm may be needed if a collision occurs. The longest time interval is the Extended InterFrame Spacing (EIFS) time interval. EIFS is used only by a station that has received a bad or unknown frame to report the bad frame.
Thus, to allocate bandwidth in conventional WLANs, the stations must contend with substantial processing overhead to compete for available bandwidth when there is no central control (e.g., in DCF mode), or must make requests for bandwidth to the AP and wait for the allocation via polling (e.g., in PCF mode). Consequently, both the DCF and PCF modes reduce throughput, especially in upstream traffic, due to the overhead involved with allocating bandwidth amongst the stations.